1. Field of the Invention
The present invention relates to a contactless power transmission system that performs power transmission contactlessly.
2. Description of Related Art
A technique has been developed in which power output by a power source is transmitted contactlessly to a load without the load and the power source being directly connected. This technique is commonly called contactless power transmission or wireless power supply. This technique has been adapted for supplying power to mobile phones, household appliances, electric automobiles, automated guided vehicles (AGVs), and the like.
With contactless power transmission, power is transmitted contactlessly from a power transmission apparatus connected to a high-frequency power source device, to a power reception apparatus connected to a load. A power transmission coil is included in the power transmission apparatus, and a power reception coil is provided in the power reception apparatus. Contactless power transmission is performed due to the power transmission coil and the power reception coil being magnetically coupled.
In the case where the load is a DC load such as a battery, a rectifying circuit is included in the power reception apparatus, and the AC current output from the power reception coil is converted into a DC current by the rectifying circuit. If the AC current output from the power reception coil is large, the current that flows in the diodes included in the rectifying circuit increases, and the diodes break down in some cases. In order to distribute the current that flows in the diodes, it is conceivable to use multiple diodes connected in parallel. However, if the AC current output from the power reception coil is a high-frequency current of 6.78 MHz to 40.68 MHz, it is difficult to balance the current that flows in the diodes connected in parallel due to the influence of the impedance of the wires, and the like, and therefore it is difficult to realize parallel connection.
In the case of using a rectifying circuit in which diodes are not connected in parallel, in order to suppress the current that flows in the rectifying circuit, there is a method of providing multiple power reception coils in the power reception apparatus and connecting rectifying circuits to the respective power reception coils. In this case, the power output by the high-frequency power source device is received by being distributed to the multiple power reception coils, and therefore the current that flows in the rectifying circuits can be dispersed. For example, Japanese Patent No. 4293854 discloses a contactless power supply apparatus that includes multiple power reception units each including a resonance circuit composed of a power reception coil and a resonance capacitor, a rectifying circuit, and a smoothing circuit, the outputs of the power reception units being connected in parallel and output to a load.
However, in the case of using the contactless power supply apparatus disclosed in Japanese Patent No. 4293854, the power reception units are equivalent to (i.e., serve as) constant voltage sources, and therefore in order to supply the output power of the power reception units to the load, the output voltages of the power reception units need to be made approximately the same. Accordingly, the specifications of the power transmission units and the specifications of the power reception units need to be the same. Also, the coupling coefficient for the power transmission coil and the power reception coil needs to be the same in each power reception unit. However, if the positional relationship (e.g., distance or the like) between the power transmission coils and the power reception coils changes, the coupling coefficients also change.